<<<<   Index

Audio limiter.

This circuit will limit the level of an audio signal to a certain maximum.
If the input level exceeds a certain value, the output level is kept constant.
The circuit can be used to prevent audio overload in transmitters, audio webcast systems, recording devices, etc. while still having a high level of audio input.
This circuit uses the NE572, this is a programmable audio compander made by Philips (NE572 datasheet).

Figure 1: Circuit diagram of the limiter.
(Update: I have now increased the value of C5 to 100 μF, which increased the release time of the limiter, and for certain kind of music improves the audio quality)

The opamps IC1, IC2 and IC3 are of type; TL072.

The NE572 IC is designed to work with nominal audio levels of 100 mVRMS.
Voltage divider R1 / R2 reduces the line level at the input ( 700 mVRMS ) to 100 mV RMS
Via C2 and C3 the signal is split in two directions.

On part of the signal enters the NE572 via pin 3, this is the rectifier input, this pin measures the average input signal level (not the peak signal level).
The rectifier circuit provides a DC voltage depending on input signal strength.
The DC voltage is adjusting the resistance between pin 5 and 7 of the NE572, and this is the feedback path for opamp IC2.

The rectifier circuit has a certain attack and release time, depending on the value of C4 (attack time) and C5 (release time).
With the given values, attack and release time are respectively about 20 ms and 200 ms.

IC2 is an inverting amplifier, the gain is depending on the components in the feedback path (between pin 1 and 2).
R8, R9, C9 is the DC feedback path, this has no influence on audio signal gain.
Capacitor C8 prevents IC2 from oscillations.

Diode D1 and D2 limits the amplitude for very short and high input peaks, shorter then the attack time of the circuit.
These peaks are for instance ticks in old vinyl records.
Without D1 and D2 the circuit tends to make such peaks even worse then at the input, making the limiter output voltage clip to the supply voltage.
With normal audio input, D1 and D2 are not conducting, because audio levels are well below the 0.6 Volt needed for the diodes to conduct.

The audio signal gain of IC2 is depending on the resistance between between pin 5 and 7 of IC4.
This resistance is adjusted by the rectifier circuit in such a way, that the output of IC2 is limited to a certain maximum.

Opamp IC3 increases the 100 mVRMS signal level (out of IC2) up to line level (about 700 mVRMS).

Figure 2: Picture of the limiter. This is a stereo version

Making a stereo limiter

To make a stereo limiter, you have to build the circuit twice.
The NE572 has two identical circuits in one package, so one for the left channel and one for the right channel.
Use the pin numbers in the circuit diagram for the left channel, and for the right channel change pin numbers as follows:

Left - Right
2    -    14
3    -    13
4    -    12
5    -    11
6    -    10
7    -    9
Note: pin 1 and 15 are not connected.


The TL072 IC's also have two circuits in one package.
Change pin numbers as follows:

Left - Right
1    -    7
2    -    6
3    -    5

The power supply

The circuit needs a plus and minus 12 volt power supply.
I used the following circuit.

Figure 3.

The power consumption of the limiter is quite low, about 10 mA, so a small transformer of about 1 Watt is enough.
The voltage regulators don't need a heatsink.

The limiter characteristics

The maximum gain of the limiter circuit is depending on the value of resistor R3, with this resistor we can adjust the maximum gain for low signal levels.
The next graph is showing this:

Figure 4: Gain of the limiter circuit as function of the value of R3.

Some gain for weak signals can be helpful to automatically increase the volume for too weak input signals.
On the other hand, to much gain also increases the level of unwanted weak signals like noise and hum, to a audible level.
I used the value: R3 = 220 KΩ , setting the maximum gain to +6 dB.
 

In the next graph, the output voltage of the circuit is shown as function of the input voltage.
We see the output voltage is limited to about -3 ....-4 dBV.
This test is done with a sine wave input of 1 kHz.

Figure 5: Output voltage as function of the input voltage. Test signal is 1 kHz sine wave.

Output voltage of the limiter

The following table gives the output voltage (peak value) for a given input voltage.
The input signal was a 1 kHz sine wave.
To get the RMS value, divide the peak value by 1.414

We see, the output limits to about 900 mV peak.
However with normal audio signals, the peaks will be higher.
Typically 2000 mVpeak for music signals, and 3000 mVpeak for a speech signal.
This is because the circuit reacts to the average input signal level, and not to peak signal level.

Linearity of the circuit.

To test the linearity of the circuit, I used a 1 kHz triangle wave at the input, and measured with an oscilloscope both input and output.

Figure 6: Linearity at high level input. Upper: output of the limiter 1 Volt per division. So 2.3 Volt p-p   Lower: input of the limiter. 5 Volt per division. So 10 Volt p-p

Figure 6: Linearity at low level input. Upper: output of the limiter. 0.2 Volt per division. So, about 0.5 Volt p-p Lower: input of the limiter. 0.1 Volt per division. So, 0.2 Volt p-p

Conclusion, in this test there is no visible non linearity of the output signal.

Modifications to the output stage.

If you want to reduce the signal level at the limiter output, you can do this by reducing the value of R13.
Or make the output level variable by replacing R13 with a potentiometer of 25 kΩ ( see next circuit diagram).

Figure 7.

When switching on the power supply for the limiter, the output voltage can clip to the 12 Volt supply rail.
These peaks can be limited to 4 Volt by adding two 3.3 Volt zenerdiodes at the output (D3 and D4).
During normal operation these zenerdiodes will never conduct, and they have no influence on audio quality.

<<<<   Index